As is generally known, a Karman vortex flow meter produces a frequency output which corresponds to a vortex velocity (number of Karman vortices) which is proportional to the intake air flow to be detected. Methods of injection feeding a fixed quantity of fuel to an engine synchronously with a frequency output proportional to the intake airflow have been proposed in Japanese Utility Model Laid Open No. 133919/1978, and Japanese Patent Laid Open No. 5448/1979.
However, in such Karman vortex flow meters, the Karman vortex frequency f is related to the width d of the vortex generating body, and the airflow velocity v, by the equation f=St.sub.v/d. In this equation, St is a constant known as the Strouhal number. This Strouhal number, as shown in FIG. 1, varies in accordance with the Reynolds number, Re, which is proportional to the product of the abovementioned d and v. Accordingly, it is necessary to compensate for the variations in the Strouhal number when fuel is injection fed to engine synchronously with the abovementioned Karman vortex frequency f.
On the other hand, the fuel injection frequency is proportional to the intake airflow, so during idling of the engine, when the intake airflow is small, the injection frequency becomes extremely low, and the idling becomes unstable. Accordingly, for said idling state, consideration could be given to increasing the airflow velocity v, or; to reducing the width d of the vortex generating body, in order to increase the Karman vortex frequency f. However, increasing the airflow velocity v also increases pressure losses, and reducing the width d of the vortex generating body results in the size of the Karman vortices produced being made smaller, making it difficult to detect the Karman vortices, while pulsation of the intake air might disturb the generation of Karman vortices, making it impossible to detect then with accuracy.